1. Field of the Invention
The present invention relates generally to a PCM signal transmission system, and is directed more particularly to a method and apparatus for processing sequentially transferred digital information words.
2. Description of the Prior Art
British Pat. No. 1,481,849 discloses a system in which an error detecting signal is added to each signal. A plurality of such signals, including the added error detecting signals, form a block. An error correcting signal, such as a parity signal, is added to each block. When an error is detected in one information signal in one block at the receiving end, all bits of the signal containing an error are set to zero and the remaining correctly received signal and the error correcting signal in the block are added by modulo 2 addition to provide a correct signal to replace the one containing an error. When more than 2 signals in one block contain errors due to, for example, burst errors which typically endure for longer than the transmission time of one word, the resulting error in two or more contiguous words in a block cannot be corrected.
In an error correcting system applied in the field of computers as disclosed in U.S. Pat. No. 3,697,948, an encoding and decoding system provides information as to which bytes are in error and extends the error correcting capability of the system to two bytes of data containing error. The transmitted message comprises k bytes of data (D.sub.0,D.sub.1,D.sub.2, - - - D.sub.k-1), each having b bits, plus two check bytes C.sub.1 and C.sub.2, each having small b bits. The message is encoded by computing the check bytes according to the following relationship: EQU C.sub.1 =I D.sub.0 .sym.I D.sub.1 +- - - .sym.I D.sub.k-1 EQU C.sub.2 =I D.sub.0 .sym.T K.sub.1 .sym.T.sup.2 D.sub.2 .sym.- - - .sym.T.sup.k-1 D.sub.k-1
Wherein I is an identity matrix and T, T.sup.2, - - - T.sup.k-1 are distinct non-zero elements of a Galois Field (2.sup.b), wherein the indicated multiplication and addition are Galois Field defined operations, and wherein b is an integer &lt;1 and k is an integer 2&gt;k&gt;2.sup.b. C.sub.1 is a simple parity code and C.sub.2 is a b-adjacent code.
A decoder is effective to recover the data without error when not more than two of the bytes in the message are in error no matter how many bits in the two bytes are in error. Pointers are required to indicate the two bytes containing errors. In the absence of pointers or in the presence of a single false pointer, the decoder is effective to recover the data without error when not more than a single byte is in error no matter how many bits may be in error in the single byte. When more than 3 bytes in k bytes simultaneously contain errors due to burst error or the like during the data transmission, such errors cannot be corrected.